The present invention relates to an internal combustion engine having a three-way catalyst in an exhaust passage, which catalyst is capable of storing oxygen. More particularly, the present invention pertains to an air-fuel ratio control apparatus for an internal combustion engine, which apparatus performs feedback control for optimizing the air-fuel ratio of combustible gas mixture supplied to the engine.
A three-way catalyst is conventionally used in a vehicle internal combustion engine to clean exhaust gas by simultaneously oxidizing unburned component (HC, CO) in the exhaust gas and reducing nitrogen oxide (NOx) in the exhaust gas. Such a three-way catalyst, which is capable of storing oxygen, cleans exhaust gas by adsorbing excessive oxygen in exhaust gas when the air-fuel ratio is lean, and by releasing oxygen that compensates for a shortage of oxygen in the exhaust gas when the air-fuel ratio is rich. To improve the oxidation and reduction performance of such a three-way catalyst, an air-fuel ratio (A/F), which represents the combustion state of an internal combustion engine, needs to be controlled such that the air-fuel ratio is in the vicinity of the stoichiometric air-fuel ratio (window). Therefore, an oxygen sensor is provided in an exhaust passage for detecting whether the air-fuel ratio is richer or leaner than the stoichiometric air-fuel ratio based on the concentration of oxygen remaining in the exhaust gas. In a typical fuel injection control of an internal combustion engine, an air-fuel ratio feedback (F/B) control is performed, in which the amount fuel is corrected based on the output of the oxygen sensor.
However, since exhaust from cylinders is not sufficiently mixed at the confluence of an exhaust manifold, which is upstream of the three-way catalyst, the output of the sensor can fluctuate. Such fluctuations of the sensor output and deterioration of the sensor due to the heat of exhaust gas can degrade the control accuracy of the air-fuel ratio.
To solve the above problems, a double sensor system has been already been put to use. In a double sensor system, an additional oxygen sensor is provided downstream of a three-way catalyst. In addition to a main F/B control based on the detection result of air-fuel ratio detected by the upstream sensor, a sub-F/B control based on the detection result of air-fuel ratio by the downstream sensor is performed to improve the accuracy of the air-fuel ratio control.
In a double-sensor system, the sub-F/B control is performed based on the detection signal of the downstream oxygen sensor. To correct the fuel amount in the main F/B control, a sub-F/B correction value is computed. Based on the sub-F/B correction value, a sub-F/B learning control is performed. In the sub-F/B learning control, a learning value is computed. The computed learning value is used for compensating for a stationary difference between the stoichiometric air-fuel ratio and the engine air-fuel ratio, which stationary difference is based on the characteristics of the upstream sensor. The learning value is used in the sub-F/B control so that the engine air-fuel ratio seeks the stoichiometric air-fuel ratio. Accordingly, the exhaust emission is prevented from deteriorating.
Since a sub-F/B learning value is cleared when the supply of electricity from the battery is interrupted, learning of the sub-F/B needs to be quickly performed when the supply of electricity is resumed. Until the learning of the sub-F/B is performed in a stabilized manner, the control accuracy of the air-fuel ratio is lowered, and the exhaust emission thus deteriorates.
In order that the sub-F/B learning is stably performed, the output of the downstream sensor needs to be repeatedly switched between a value representing a rich air-fuel ratio and a value representing a lean air-fuel ratio during the execution of the sub-F/B control. Alternatively, the execution of the sub-F/B needs to be continued for a predetermined period of time or longer.
However, when a fuel cutoff (F/C) control is started in the internal combustion engine, the main F/B control and the sub-F/B control are not executed. At this time, if the three-way catalyst is not degraded and has a high oxygen storing property, the catalyst stores a significant amount of oxygen and is saturated. This causes the downstream oxygen sensor to detect a lean air-fuel ratio.
Therefore, even if the F/B control is performed to make the air-fuel ratio rich after the fuel cutoff control is stopped, the output of the downstream oxygen sensor is not easily switched from a value representing a lean air-fuel ratio to a value representing a rich air-fuel ratio because the three-way catalyst releases adsorbed oxygen. As a result, the sub-F/B learning value, which is computed based on the sub-F/B correction value in the sub-F/B control, does not have a proper value that the sub-F/B learning value should originally have.
A fluid transmission device having a lockup clutch is located between an internal combustion engine of an automobile and an automatic transmission. As disclosed in Japanese Laid-Open Patent Publication No. 9-292019, such a lockup clutch performs a deceleration lockup slip control when a vehicle is decelerating, thereby maintaining a slipping state between the automatic transmission and the internal combustion engine. Accordingly, the engine speed is maintained to or above the speed for the fuel cutoff (F/C). One of the reasons for engaging the lockup clutch in upshifting of the automatic transmission accompanying deceleration of the engine is to continue the fuel cutoff (F/C) as long as possible to maximize the improvement of the fuel economy.
If a three-way catalyst is placed in the exhaust passage of such an internal combustion engine and the air-fuel ratio F/B control is performed, frequent execution of the deceleration lockup slip control accompanying the fuel cutoff (F/C) causes the sub-F/B learning value to be unstable. Particularly, when a sub-F/B learning value is learned after the learning value is cleared, the execution of the fuel cutoff (F/C) control accompanying the deceleration lockup slip control prevents the sub-F/B learning from being performed.
That is, as shown in FIG. 12, when the execution of the sub-F/B control is continued for a predetermined period during the sub-F/B control at time t10, the sub-F/B learning is determined to be stable. Then, the sub-F/B learning value is renewed from an initial value (zero) by a predetermined amount based on the sub-F/B correction value. The sub-F/B correction value is reduced by an amount that corresponds to the correction amount of the sub-F/B learning value. During the deceleration lockup slip control, if the fuel cutoff control (F/C) is repeated in a short period after time t11, each execution of F/C causes the output of the oxygen sensors to seek a value representing a lean air-fuel ratio. Even if the air-fuel F/B control is resumed after the F/C is stopped, the output of the oxygen sensors will not change before a predetermined time elapses after the F/C is stopped because the three-way catalyst releases oxygen. Therefore, the sub-F/B correction value will not be renewed after time t11. Further, since the F/C is repeated in a short period of time and the F/B control is not executed for a predetermined period or longer, the sub-F/B learning is not performed. Also, the sub-F/B learning value is not renewed after time t11. This causes the sub-F/B learning value to be a value different from a proper value that the sub-F/B learning value should originally has. Therefore, even if the main F/B control and the sub-F/B control are executed based on the learning value, the engine air-fuel ratio cannot be controlled to seek a value close to the stoichiometric air-fuel ratio. This can deteriorate the exhaust emission.